The present invention relates to a system for controlling air-fuel ratio of mixture for an automotive engine, and more particularly to a system for detecting an activated state of an O.sub.2 -sensor for restarting the engine.
The O.sub.2 -sensor does not produce a normal output voltage before the temperature of the sensor body rises above an activation temperature. Accordingly, an air-fuel ratio control operation using a feedback signal from the O.sub.2 -sensor must be started after the activation of the O.sub.2 -sensor.
The activation of the O.sub.2 -sensor can be determined by the fact that the output voltage of the O.sub.2 -sensor exceeds a predetermined reference value. For example, Japanese Patent Application Laid Open No. 58-8246 discloses a control system in which the sum of the output voltage of the O.sub.2 -sensor and a standard voltage is compared with a reference voltage.
However, there is a problem in the detection by the output voltage of the O.sub.2 -sensor. Namely, if the engine is re-started in the condition when the temperature of the O.sub.2 -sensor is in a low temperature range, the O.sub.2 -sensor produces a high error voltage for a period of time in spite of an inactivated state. In order to avoid such a malfunction, if the reference voltage is set to a high value, a long time elapses before staring the feedback operation in normal warming up of the engine.